Agreed, you don't need light in a FW
sump, unless you want some plants/algae for nitrate absorption (in that case your sump is now called a fuge).
However, it is handy to have a light in the cabinet. I wired in a small Florescent fixture that I can turn on when doing maintanance (and when looking for the small parts that I shove in the cabinets for storage!).
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.